In an air conditioner for automobile, there are an outside air lead-in mode for performing air conditioning operation while leading in the outside air from the outside of a vehicle and an inside air circulating mode for circulating the air in the vehicle to perform air conditioning operation without leading in the outside air from the outside of the vehicle. It is possible to prevent deterioration in an inside air quality by usually setting the outside air lead-in mode and, for example, when an exhaust gas odor from the outside of the vehicle is disturbing, switching the outside air lead-in mode to the inside air circulating mode to block intrusion of the exhaust gas.
In recent years, an air conditioner that automatically performs such mode switching of the outside air lead-in mode and the inside air circulating mode by detecting an exhaust gas component in the lead-in outside air has already been proposed (see, for example, Patent Document 1).    Patent Document 1: Japanese Patent Laid-Open No. 2004-268792